Tubing used in a motor vehicle as a fuel line, brake line, vacuum line, coolant hose, or the like is typically made of one or more layers of synthetic resin as a fairly stiff tubing or hose. It is designed to have certain predetermined characteristics--hardness, strength, elongation on break, notch impact strength--and can generally be counted on to give good service down to about -40.degree. C. These characteristics are typically determined by selection of the resin making a one-layer tube or the resins in a multilayer one. Reference should be made to U.S. Pat. Nos. 5,076,329 and 5,167,259 as well as to German patent documents 3,715,251 and 3,821,723, all of E. Brunnhofer, for further details.
Typically the last stage of manufacture of such a hose or tube is a calibration step where the tubing moves through a cold vacuum-calibrating die to set its outside diameter to the exact desired size. This step has the side effect of orienting the outer layer of the tube, that is that portion of the tube about 20 .mu.m to 200 .mu.m thick that is directly contacted by the calibrating die. This unfortunately results in an outer layer that is excessively sensitive to cold. Thus when used in extremely frigid surroundings, about -40.degree. C., the hose is subject to cracking.
Thus it has been proposed in European patent application 87 400 666 filed by J. Labalg (based on a French priority of 04 Apr. 1986) to eliminate the orientation of the outer layers of the tube by flame treating the tube. Thus the finished calibrated tube is passed quickly through a flame to heat its outer surface sufficiently to relax any stresses created therein by the calibrating step without softening it enough to actually change its diameter or shape.
Such an additional step is clearly another manufacturing cost that must be added to the cost of making the tube. In addition it is fairly difficult to get the flame-relaxing just right, as too much heat deforms the tube or changes its chemical composition while not enough leaves unwanted stresses in its outer surface.
Another problem with such tubing is that when used in a vehicle that is lacquered, typically with an acrylic lacquer, some of this lacquer gets on the tubing. The lacquer solvent is much more brittle than the tubing, particularly at extremely low temperatures. Thus when the tubing flexes somewhat when very cold, the lacquer cracks and the energy of this cracking is transmitted to the underlying tubing directly under the newly formed crack, forming a crack in this tubing at this location. This is even true in tubing subjected to flame recrystallization as described above.